The efficiency of internal combustion engines is often expressed in terms of thermal efficiency, which is a measure of an engine's ability to convert fuel energy into mechanical power. Conventional internal combustion engines with reciprocating pistons typically have relatively low thermal efficiencies. Conventional automobile engines, for example, typically have thermal efficiencies of about 0.25, which means that about seventy-five percent of the fuel's energy is wasted during engine operation. Specifically, about forty percent of the fuel's energy flows out the exhaust pipe as lost heat, while another thirty-five percent is absorbed by the cooling system (i.e., coolant, oil, and surrounding air flow). As a result of these losses, only about twenty-five percent of the fuel's energy is converted into usable power for moving the car and operating secondary systems (e.g., charging systems, cooling systems, power-steering systems, etc.).
There are a number of reasons that conventional internal combustion engines are so inefficient. One reason is that the cylinder head and walls of the combustion chamber absorb heat energy from the ignited fuel but do no work. Another reason is that the ignited fuel charge is only partially expanded before being pumped out of the combustion chamber at a relatively high temperature and pressure during the exhaust stroke. An additional reason is that reciprocating piston engines produce very little torque through much of the piston stroke because of the geometric relationship between the reciprocating piston and the rotating crankshaft.
While some advancements have been made in the field of piston engine technology, it appears that the practical limits of piston engine efficiency have been reached. The average fuel economy of new cars, for example, has increased by only 2.3 miles-per-gallon (mpg) in the last 20 years or so. More specifically, the average fuel economy of new cars has increased from 26.6 mpg in 1982 to only 28.9 mpg in 2002.
Although a number of alternatives to the conventional internal combustion engine have been proposed, each offers only marginal improvements. Hybrid vehicles, for example (e.g., the Toyota Prius), and alternative fuel systems (e.g., propane, natural gas, and biofuels) still use conventional reciprocating piston engines with all of their attendant shortcomings. Electric cars, on the other hand, have limited range and are slow to recharge. Hydrogen fuel cells are another alternative, but implementation of this nascent technology is relatively expensive and requires a new fuel distribution infrastructure to replace the existing petroleum-based infrastructure. Accordingly, while each of these technologies may hold promise for the future, they appear to be years away from mass-market acceptance.